Ophthalmic surgery frequently involves the removal of fluid and/or tissue from the eye via a surgical hand piece or connector to a cannula. The surgical hand piece includes a tip, an aspiration line, an irrigation line and associated electronics. The aspiration and irrigation lines are connected with the tip. The tip has the appropriate devices for the procedure being performed. Fluid and/or tissue is removed via the aspiration line, while the irrigation line provides fluid to the eye. The surgical hand piece or cannula is also connected to and controlled by a console. The aspiration line is connected to the tip and to tubing coupled to a vacuum pump controlled by the console. Similarly, the infusion line is connected to the tip and to tubing coupled to a fluid source. The vacuum pump and fluid source are typically remote from the surgical hand piece and may be part of the console. For example, the vacuum pump and other portions of the console may be several feet from the surgical hand piece. In operation, the tip is inserted into the patient's eye. A vacuum is applied to the aspiration line in order to remove material from the eye. Fluid is also provided to the eye via the infusion line, allowing the intraocular pressure of the eye to be maintained.
Although the ophthalmic surgery may be performed, there is a danger of the eye collapsing during surgery. For example, in cataract surgery, the cataract is broken up and removed from the patient's eye. The console controls piezoelectric transducers in the surgical hand piece that cause a cutting tip to oscillate at ultrasonic frequencies, breaking up the cataract. A large diameter port in the tip is coupled to the aspiration line. The vacuum pump, typically peristaltic, applies a large negative pressure to tubing connected to the aspiration line in order to remove the dense cataract tissue. The pieces of the cataract, vitreous humor and other tissue flow out of the eye through the port and aspiration line. Fluid also flows into the eye through the irrigation line. However, tissue may occlude the port. At some point, the tissue blocking the port breaks up and passes through the aspiration line. As a result, there is a sudden, large increase in the outflow of material from the eye and a corresponding high vacuum in the aspiration line. The inflow of fluid generally cannot keep up with this flow. The eye may then collapse around the tip, which is highly undesirable.
A similar issue exists in the extraction of silicone oil from the eye. In some surgeries, a viscous fluid, such as silicone oil, may be injected into the eye to support the eye during healing. Later, the silicone oil is extracted. In this case, the hand piece acts as a syringe. The tip includes a cannula that is inserted into the patient's eye. A piston resides in the aspiration line. A vacuum pump controlled via the console applies a negative pressure to the aspiration line and, therefore, the portion of the piston distal from the cannula. In response, the piston moves along the aspiration line and the silicone oil may be extracted through the cannula and into the aspiration line. Simultaneously, the infusion line provides fluid to the eye to maintain the intraocular pressure of the eye. The viscosity of the silicone oil is typically several orders of magnitude greater than that of the vitreous humor. Consequently, when the last of the silicone oil is removed, there is a rapid change in the viscosity of the fluid entering the tip. If the vacuum applied to the piston is not changed, the piston will suddenly increase its speed. A relatively large volume of fluid may then be rapidly extracted from the eye. The inflow from the irrigation line generally cannot keep up with this rapid outflow. Again, the eye may collapse.
In order to prevent collapse of the eye in the above situations, the surgeon must rapidly decrease the vacuum applied to the aspiration line in response to seeing the pressure drop in the aspiration line or the last of the silicone oil removed. Consequently, the surgeon constantly monitors the status of the eye and must quickly respond to a change in the flow of fluid from the eye. This may be difficult. Even if the surgeon is vigilant, decreasing the vacuum in the aspiration line is still subject to human response time, which may be relatively long (over 400 hundred milliseconds). As a result, the eye may still collapse.
Accordingly, what is needed is a mechanism for assisting a physician in ophthalmic surgery in which fluid, tissue and/or other material is removed from the eye.